The present invention relates generally to hollow inorganic nanomaterials and methods for making them. More particularly, the invention relates to hollow silica nanomaterials (HSNs). Hollow inorganic nanomaterials are attracting a significant amount of attention from those skilled in the art. The increase in attention is due to the potential applications hollow inorganic nanomaterials have in areas such as bioanalysis, drug delivery, active metal encapsulation, and in acting as nano-reactors for catalysis. Researchers have synthesized a wide range of hollow spherical nanomaterials including those composed of TiO2, SnO2, Fe3O4, and SiO2.
HSNs are attractive synthetic targets. This is because of the hydrophobic nature, easy colloidal suspension formation, excellent biocompatibility, and surface functionalization accessibility offered by HSNs. Researchers have taken advantage of these properties to devise useful applications for HSNs.
Two methods are commonly used to synthesize hollow nanomaterials: hard-templating and template-free synthesis. The hard-template method generally involves: (1) the preparation of a template as a solid core; (2) surface functionalization by depositing a silica shell on the surface of the template; and (3) removal of the template by either calcination at high temperatures or selective etching. The morphology of the hollow silica nanomaterials is controlled by the template used during synthesis. The desirability of using the hard-template method however, is tempered by some drawbacks associated with it. For instance, the hard-template method is a multi-step procedure and is both time consuming and difficult to optimize for large-scale synthesis. A further disadvantage of the hard template method is that the removal of the template may cause the hollow nanomaterial to collapse. These difficulties are a function of the harsh nature of the calcination reactions and selective etching solvents used for removing the template.
An alternative to the hard template method is the template-free method. An advantage to the template-free method is that no template removal step is required. The template-free method however, presents difficulties in controlling the size, shape, monodispersity, and surface structure of the hollow silica nanomaterials.
The synthesis of HSNs remains difficult. Most applications of any technique require strict reaction conditions, in addition to high costs and tedious synthetic procedures. Moreover, although researchers have been able to synthesize rod-like materials having a hexagonal cross-section and spherical HSNs, the morphology of the HSNs is generally not tunable. As a result, synthesizing an HSN while controlling the morphology of the nanomaterial is difficult.